Conventionally, the following technologies are known for magnetic recording media.
(1) In international application PCT/JP94/01184, it is disclosed that in a magnetic recording medium formed by means of a sputtering formation method in an ultraclean atmosphere (having an attained vacuum degree of 10.sup.-9 Torr or less), it is possible to limit the amount of oxygen contained in the undercoat and the magnetic layer comprising the magnetic recording medium to 100 wt.ppm or less, and because it is possible to limit the exchange interaction of the crystalline particles comprising the magnetic layer, the magnetic characteristics (the coercive force, and the normalized coercive force) and the storage and readout characteristics (S/N) are improved.
(2) It is well known that when ferromagnetic metal layers are laminated, by applying an electrical bias to the substrate body, the magnetic characteristics are improved. In particular, it has been reported by Okumura et al. that when the ferromagnetic metal layer comprises an alloy film consisting of CoCrTa, this effect is dramatic ("Substrate bias effect on the magnetization of a Co alloy/Cr thin film," Nippon Oyo Jiki Gakkai Gaiyoushu (1991), 413, p 413, 1991).
(3) It has been made clear by Iwabune et al. that in order to achieve an increase in the recording density of magnetic recording media, a magnetic recording medium having a small surface roughness is optimal ("Dependence of reproduction output on spacing in vertical two layer media", Nippon Oyo Jiki Gakkaishi, Volume 16, p. 105, 1992). That is to say, the small surface roughness of the magnetic recording medium realizes a lessening of the distance at which the head floats. As a result, the reproduced signal increases, so that this indicates that it would be possible to conduct readout of smaller recording bits.
(4) It is commonly known that in order to achieve an increase in the recording density of magnetic recording media, it is necessary to reduce medium noise (Nm). That is to say, it is important to increase the S/Nm ratio (reproduction signal (S)), and in order to accomplish this, it is necessary to reduce the magnetization transition region produced between adjacent storage bits. A method in which the recording layer is formed from magnetic particles having a small diameter has been disclosed by Nakai et al. as a way of accomplishing this ("Effect of Microstructure on Media Noise of CoCrTa Thin Film Media Fabricated under UltraClean Sputtering Process", 1995 IEEE International Magnetics Conference Digests of the Technical Papers, JA-05, 1995).
However, when the technique described in (2) above is employed, the surface roughness of the magnetic recording medium is large, and there is a tendency for the diameter of the magnetic particles to become disarranged and chaotic. This tendency is identical when the technique described in (1) above is employed. As a result, it is difficult to produce a magnetic recording medium having a small head float distance and having low medium noise.
Accordingly, the development of a magnetic recording medium, and a manufacturing method thereof, having a small surface roughness and having a recording layer which is formed from magnetic particles having a small diameter, is desirable from the point of view of furthering an increase in the recording density of magnetic recording media.
The present invention has as an object thereof to provide a magnetic recording medium and a manufacturing method thereof, comprising a recording layer comprising magnetic particles having a small diameter and having a small surface roughness, and having a high coercive force.